Hi Akira Sato,
It sounds like you don't the Photos library set up for the Photo Browser in iMovie to access it. Here is an alternate way to bring your still images into iMovie.
When you open the Photos application on your computer, do you see the image that you want to bring into iMovie? If so:
- Drag the image from Photos to your desktop.
- In iMovie choose File > Import Media from the Menu bar.
- Select Desktop > select the image > then choose Import Selected.
If you have more than one image that you want to bring into iMovie:
- Create a folder first on the desktop.
- Drag the images you want into the folder
- Import the entire folder to iMovie using the steps above.
http://help.apple.com/imovie/mac/10.1/#/mov6fa10197d
Select a file, Command-click to select multiple files, or select a folder of files, and drag the selection from the Finder to an event in the Libraries list in iMovie.
The clip or clips appear in the event.
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If the media you want to import is on an external storage device, connect it to your Mac and turn it on.
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To open the Import window, click the Import button in the toolbar.

If you don’t see the Import button, click the Media button in the toolbar, and then click the Import button.

If the Image Capture, Photos, or iPhoto window appears, close the window.
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In the Devices section of the Import window sidebar, select the device you want to import media from.
The contents of the device appear in a list at the bottom of the Import window. You can select clips individually in list view, and a preview of the selected clip appears at the top of the Import window.

You can do any of the following:
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Sort clips in list view by data such as duration, creation date, size, and so on: Click the column headings at the top of list view.
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Add a folder of frequently used media to the Favorites section: Drag a folder from the list on the right to Favorites in the sidebar.
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Remove a folder from Favorites: Control-click the folder and choose Remove from Sidebar from the shortcut menu.
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To preview video, move the pointer over the preview at the top of the Import window and click the Play button
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You can also click the Previous or Next button to go to the previous or next clip, or click and hold the Previous or Next button to rewind or fast-forward.

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To specify where the imported media will be stored, do one of the following:
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Choose an existing event: Click the “Import to” pop-up menu at the top of the Import window and choose the event.
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Create a new event: Click the “Import to” pop-up menu, choose New Event, type a name for the new event, and click OK.
To learn more about events, see Organize media and events.
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Do one of the following:
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Import all clips: Click Import All.
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Import selected clips: Command-click each clip you want to import, and click Import Selected (the Import button changes its name).
The Import window closes, and your clips appear in the event. Depending on the length of each clip and the number of clips you’re importing, you may see a progress indicator in the upper-right corner of the window while the clips are being imported.

You can continue to work in iMovie while the import finishes.
Happy Holidays!